<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="./lib/vue.min.js"></script>
</head>
<body>
    <div id="app">

         <!-- 分析 -->
         <!-- 1，我们要监听 文本框数据的改变，这样才能知道 什么时候去拼接fullname -->
         <!-- 2.如何监听到 文本框数据的改变 -->


        <input type="text" v-model = "firstname" > +
        <input type="text" v-model = "lastname"> =
        <input type="text" v-model = "fullname">
    </div>
    
    <script>
        var vm = new Vue({
            el:"#app",
            data:{
                firstname: '',
                lastname: '',
                fullname: ''
            },
            methods: {},
            watch:{ //使用这个属性 可以监视 data 中指定数据的变化，然后触发 watch 中对应的 function 处理函数
                firstname: function(newVal, oldVal) {
                    //this.fullname = this.firstname + "-" + this.lastname
                    this.fullname = newVal + "-" + this.lastname
                },
                lastname: function(newVal) {
                    this.fullname = this.firstname + "-" + newVal
                }
            }
        })
    </script>
</body>
</html>